Original Research Article

Efficacy of wheat straw mulching in reducing soil and water lossesfrom three typical soils of the Loess Plateau, China

Abbas E. Rahma a, b, David N. Warrington d, Tingwu Lei c, d, *

a College of Agricultural Studies, Department of Agricultural Engineering, Sudan University of Science and Technology, Shambat, Khartoum, Sudanb College of Water Conservancy and Civil Engineering, Shandong Agricultural University, Taian Shandong, PR Chinac College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, PR Chinad State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Science andMinistry of Water Resources, Yangling, Shaanxi Province 712100, PR China

a r t i c l e i n f o

Article history:Received 5 May 2019Accepted 9 August 2019Available online 14 August 2019

Keywords:Straw mulchSoil lossesRunoffRain simulator

a b s t r a c t

Mulching the soil surface with a layer of plant residue is considered an effective method of conservingwater and soil because it increases water infiltration into the soil, reduces surface runoff and the soilerosion, and reduces flow velocity and the sediment carrying capacity of overland flow. However,application of plant residues increases operational costs and so optimal levels of mulch in order toprevent soil and/or water losses should be used according to the soil type and rainfall and slope con-ditions. In this study, the effect of wheat straw mulch rate on the total runoff and total soil losses from60-mm simulated rainstorms was assessed for two intensive rainfalls (90 and 180mmh�1) on threeslope gradients typical conditions on the Loess Plateau of China and elsewhere.

For short slopes (1m), the optimal mulch rate to save water for a silt loam and a loam soil was0.4 kgm�2. However, for a clay loam soil the mulch rate of 0.4 kgm�2 would be optimal only under the90mmh�1 rainfall; 0.8 kgm�2 was required for the 180mmh�1.

In order to save soil, a mulch rate of 0.2 kgm�2 on the silt loam slopes prevented 60%e80% of the soillosses. For the loam soil, mulch at the rate of 0.4 kgm�2 was essential in most cases in order to reducesoil losses substantially. For the clay loam, 0.4 kgm�2 may be optimal under the 90mmh�1 rain, but0.8 kgm�2 may be required for the 180mmh�1 rainstorm. These optimal values would also need to beconsidered alongside other factors since the mulch may have value if used elsewhere. Hence doublingthe optimal mulch rate for the silt loam soil from 0.2 kgm�2 or the clay loam soil under 90mmh�1

rainfall from 0.4 kgm�2 in order to achieve a further 10% reduction in soil loss needs to be assessed inthat context. Therefore,. Optimal mulch rate can be an effective approach to virtually reduce costs or tomaximize the area that can be treated. Meantime, soil conservationist should be aware that levels ofmulch for short slopes might not be suitable for long slopes.© 2019 International Research and Training Center on Erosion and Sedimentation and China Water andPower Press. Production and Hosting by Elsevier B.V. This is an open access article under the CC BY-NC-

ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Soil erosion is a severe problem for most cultivated land in theworld, and particularly on the Loess Plateau of China. The LoessPlateau is located in the upper and middle reaches of the YellowRiver (from 100� 54' to 114� 33'E and 33� 43' to 41�16' N). It covers atotal area of 624,000 km2 and the soils are derived from thick

ancient loess deposits (Gao et al., 2016). Over 60% of the area of theLoess Plateau is subject to great soil and water losses, with a meanannual soil loss of 2000e2500 t km�2. Soil erosion by water hasbeen the major cause for the losses of land nutrients and produc-tivity. In recent years, off-site problems such as river/channel andreservoir sedimentation and waters pollution by sediment-bornechemicals have also become a major concern (Poesen,Nachtergaele, Verstraeten, & Valentin, 2003; Udawatta, Motavalli,& Garrett, 2004; Wu, Mersie, Atalay, & Seybold, 2003). The severesoil erosion in the Loess Plateau is mainly caused by erosive rainfallevents (Wu, Liu, & Ma, 2016a; Zhang & Zhu, 2006).

Soil erosion bywater begins with the production of runoff water

* Corresponding author. College of Water Resources and Civil Engineering, ChinaAgricultural University, Beijing 100083, PR China.

E-mail address: leitingwu@cau.edu.cn (T. Lei).

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International Soil and Water Conservation Research

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https://doi.org/10.1016/j.iswcr.2019.08.0032095-6339/© 2019 International Research and Training Center on Erosion and Sedimentation and China Water and Power Press. Production and Hosting by Elsevier B.V. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

International Soil and Water Conservation Research 7 (2019) 335e345

during a rainstormwhen the infiltrability of the soil becomes lowerthan the rainfall intensity. One of the main factors affecting infil-trability is seal formation, which reduces the hydraulic conductivityof the soil surface layer. Seal formation on freshly cultivated fieldbegins with the breakdown of surface clods and aggregates by bothphysical forces and physicochemical processes (Agassi, Shainberg,& Morin, 1981; Lado, Ben-Hur, & Shainberg, 2004; Assouline &Ben-Hur, 2006). The physical forces are primarily produced byraindrop impact, which also compact the soil surface, and slaking.Physicochemical dispersion is determined by the electrolyte con-centration of the rainwater and by the concentration of elements inthe soil, particularly sodium (Ma, Li, Cai, &Wang, 2014). Dispersionof clay results in free clay entering the surface soil pore system andpartially blocking the pores.

Slaking is caused by the explosive force of escaping air that wasentrapped under pressure inside dry aggregates during wetting(Emerson, 1967; Fajardo, Mcbratney, Field, & Minasny, 2016;Panabokke & Quirk, 1957; Yoder, 1936; Zaher & Caron, 2008). It ismost severe when dry aggregates are rapidly wetted. The forcegenerated by slaking depends on the volume of air entraption in-side the aggregates, the rate of clod wetting (Loch, 1994; Zaher,Caron, & Ouaki, 2005; Chenu, Le Bissonnais, & Arrouays, 2000;Fan, Lei, Shainberg, & Cai, 2008; Han et al., 2016; Lado et al., 2004),and the shear strength of wet aggregates (Nearing & Bradford,1985; Fatte et al., 2011). Slaking depends on aggregate stability,which is directly related to organic matter, sesquioxides and claycontents (An, Darboux, & Cheng, 2013; Barth�es et al., 2008; Angers& Kay, 1999; Kemper& Koch, 1966; Le Bissonnais& Arrouays, 1997;Norton, Mamedov, & Levy, 2006; Puget, Chenu, & Balesdent, 1995).Mulching is referred to as the agronomic practice of covering thesoil surface with straw for soil and water conservations and tofavour plant growth (Jord�an, Zavala, & Mu~noz-Rojas, 2011).

The effectiveness of mulching in reducing runoff and soil losscan be attributed to three main aspects. Firstly, using mulch toprotect the soil surface from the direct impact of raindrops, reducessplash erosion and soil detachment and, thereby, limiting theavailability of detached soil readily being transported by runoff(Schwab, Frever, Edminster, & Barnes, 1993; Lal, 1979; Gholami,Sadeghi, & Homaee, 2013; Cook, Valdes, & Lee, 2006; García-Orenes et al., 2009, 2012; Keesstra et al., 2016; Mwango et al.,2016; Prosdocimi et al., 2016a, b) as well as reducing soil surfacecrusting, sealing and compaction (Cook et al., 2006; Jord�an, Zavala,& Gil, 2010; Montenegro, Abrantes, de Lima, Singh,& Santos, 2013a,b; Zonta, Martinez, Pruski, Silva, & Santos, 2012). It is thereforeconsidered to be an effective way to control soil erosion by water(Gabet, Reichman, & Seabloom, 2003; Zhang, Liu, Zhang, & Xie,2003). Secondly, mulch increases the hydraulic roughness of thesoil surface, thereby reducing surface flow velocity which reducessoil detachment and the carrying capacity of the overland flow(Montenegro et al., 2013a, b; Shi et al., 2013; Foster & Meyer, 1975;Cruse, Mier,&Mize, 2011; Jord�an et al., 2010; Miyata, Kosugi, Gomi,& Mizuyama, 2009; Rahma et al., 2013). Thirdly, mulch entrapswater and soil (Cerd�a et al., 2016; Foltz & Wagenbrenner, 2010;Groen & Woods, 2008; Pannkuk & Robichaud, 2003; Prats,Wagenbrenner, Malvar, Martins, & Keizer, 2016b, 2012;Robichaud et al., 2013), especially in the beginning of a rainfallevent when the mulch is dry and its capacity to retain water andsoil particles is the highest.

Many studies have evaluated the use of various plant residuemulches on soil erosion (Hou&Du,1985; Luo, Bai,& Song,1990; Jin,Shi, & Hou, 1992, pp. 60e74; Achmad, Anderson, Gantzer, &Thompson, 2003; Cook et al., 2006; Prosdocimi et al., 2016a;Sadeghi, Gholami, Homaee, & Khaledi Darvishan, 2015a; Shi et al.,2013).

Factors affecting soil loss include mulch cover, rainfall intensity

and rainfall duration, and slope gradient (Auerswald, Kainz, &Fiener, 2003; Francis & Thornes, 1990; Jin et al., 2009; Khan,Monke, & Foster, 1988; Lattanzi, Meyer, & Baumgardner, 1974;Sadeghi, Gholami, Sharifi, Khaledi Darvishan, & Homaee, 2015b;Smets, Poesen, & Knapen, 2008b).

Reports have indicated that mulching is one of the most costeffective means of crop residue usage (Dickey, Shelton, Jasa, &Peterson, 1985; Shelton, Dickey, Hachman, Steven, & Fairbanks,1995). Even so, mulch is of use for other purposes, such as feedfor animal, fuel for cooking, or as a building material. Therefore,excess use of mulch to reduce water and soil losses is desirable.

Applying straw mulch always reduced water and soil losses ascompared to un-mulched soil. However, the mulch rates requiredto reduce soil and water losses to an optimal level depended on thesoil type and the rainfall and slope conditions. The rates could alsodepend on whether the objective was primarily to reduce soil orwater losses. However, although the advantageous effects ofmulching with crop residues are known, further research is neededto quantify these effects, particularly in areas where soil erosion bywater represents a severe threat. Arguably, there are still someuncertainties in the literature about how to maximize the effec-tiveness of straw mulch for reducing soil and water loss rates.

First, the choice of vegetative residue cover type is essentially;this choice drives the application rate, cost and, consequently,effectiveness of mulching (Robichaud et al., 2013; Smets, Poesen, &Bochet, 2008a, 2008b; Beyers, 2004; Erenstein, 2003; Lal, 1976;Prats et al., 2012). Second, the appropriate application rate isanother significant factor that substantially influences the effec-tiveness of mulching in reducing soil and water losses (Jord�an et al.,2010; Lal, 1984; Lattanzi et al., 1974; Meyer, Wischmeier, & Forster,1970; Mulumba& Lal, 2008; Prosdocimi et al., 2016a) as well as thepercentage of area covered by mulch (Adekalu, Olorunfemi, &Osunbitan, 2006; Harold, 1942; Lal, 1977; Norton, Cogo, &Moldenhauer, 1985).

Most of the studies focusing on the efficiency of mulching toreduce runoff and erosion were carried out in the field. Theyinvolved natural rainfall conditions (Cook et al., 2006; Martinez-Raya, Duran-Zuazo, & Francia-Martinez, 2006; Mupangwa,Twomlow, Walker, & Hove, 2007; Prats et al., 2012, 2014, 2016a,b; Robichaud et al., 2013; Are et al., 2011; Bhatt & Khera, 2006;Cawson, Sheridan, Smith,& Lane, 2013) as well as simulated rainfall(Cerd�a,1997; Cerd�a et al., 2016; Groen&Woods, 2008; Jord�an et al.,2010; Mayor, Bautista, & Bellot, 2009; Montenegro, de Lima,Abrantes, & Santos, 2013b; Robichaud et al., 2013) and appliedconcentrated flow from upslope (Harrison, Stubblefield, Varner, &Knapp, 2016; Robichaud et al., 2013). Field studies research and,in particular, those under natural rainfall conditions, are typicallyhigh time-consuming and need demanding in resources and facil-ities, as they often require many years to obtain representativeresults of the aimed soil and rainfall conditions (Lal, 1994). How-ever, experiments under laboratory conditions using soil flumeshave been used to study runoff and soil erosion processes (Marzen,Iserloh, de Lima, & Ries, 2016; Prats, Abrantes, Coelho, Keizer, & deLima, 2018; De Lima, Singh, & de Lima, 2003, 2013), to determinethe impacts of mulching (Foltz & Wagenbrenner, 2010; Gholamiet al., 2013; Montenegro et al., 2013a; Pannkuk & Robichaud,2003; Prats, Abrantes, Crema, Keizer, & de Lima, 2017, 2015; Xu,Zheng, Qin, Wu, & Wilson, 2017). The main usefulness of suchlaboratory experiments is that they allow systematic replication ofa wide range of rainfall and terrain conditions (e.g., rainfall spatialand temporal characteristics, surface slope, and soil roughness).

The objectives of this study are : (i) to determine the effect ofwheat straw mulch rate on seal formation, infiltration, runoff andsoil loss under different rainfall intensitiesy and slope conditions;(ii) to assess the efficacy of different rates of straw mulch cover in

A.E. Rahma et al. / International Soil and Water Conservation Research 7 (2019) 335e345336

reducing soil and water losses during intensive rainstorms typicalof the Loess Plateau and other areas of the world; and (iii) to sug-gest optimal levels of mulch to reduce soil and water losses ac-cording to the soil type and rainfall intensity and slope condition.

2. Material and methods

Soil was collected from three locations in Shaanxi Province (asilt loam soil from Ansai, a clay loam soil from Yangling, and a loamsoil from ChangWu) on the Loess Plateau, which represented threecommon agricultural soils that differed in texture and are situatedin the most productive part of the Plateau. Samples were collectedfrom the upper 20 cm soil layer of cultivated land at each location.The soils were air-dried to a gravimetric moisture content of lessthan 8%. Large clods weremanually broken apart and the soils werethen passed through 2mm mesh. A representative subsample ofthe soils was used for chemical and mechanical analysis. Cationexchange capacity, exchangeable sodium percentage and organicmatter content were determined according to standard methods. AMastersizer 2000 (Malvern Instruments, Malvern, England)analyzer was used to determine particle size distributions. Basicsoil properties are given in Table 1.

Steel boxes measuring 1.0m in length, 0.6m in width, and0.25m in depth, whichwere supported on amobile framework thatcould be set at various slope angles, was used to contain the soilsduring the experiments. Holes, 3mm in diameter with a spacing of10mm, in the bottom of the box allowed air to escape; a layer ofthin cloth covered the bottom of the box to prevent loss of soilthrough the holes. A funnel at the lower end of the box directedrunoff into collection buckets. With the flumes in a horizontal po-sition, the air-dried soil material was uniformly packed into theboxes, by pouring a known mass of soil into a known volume of thebox in layers of 2.5 cm thickness and tamping it down with awooden paddle so that the bulk density was 1200 kgm�3. This bulkdensity was representative of freshly tilled soils on the LoessPlateau. The surface of each layer was scored rough before addingmore soil to minimize discontinuity effects.

A rainfall simulator, similar to the one described by Meyer andMcCune (1958), at the State Key Laboratory of Soil Erosion andDryland Farming on the Loess Plateau, was used to perform theexperiments which were conducted in Yangling, China (108�240Eand 34�200N, 521m above sea level). The equipment was capable ofgenerating simulated rainfall with deionizedwater over large areas.Simulated rainfall was projected sideways from eight nozzles sit-uated 16m above the ground, and then fall vertically towards theground. Rainfall intensity was determined by valves linked topressure gauges, controlled automatically by a computer moni-toring an electronic rain gauge. The raindrop speed after calibrationmeets natural rainfall features.

Deionized water, used to simulate rainwater, was projectedsideways from six nozzles, in two rows of three arrayed over thetwo long sides of the rectangular target area (4m� 9m) Fig. 1. Thenozzles were 16m above the ground so that the raindrops attainedtheir terminal velocity, about 98% of that of natural rain. Rainfallintensities were determined by a pump that was controlled by a

computer connected to a rain gauge positioned in the center of thetarget area.

Following packing, mulch was spread uniformly over the soilsurface at the rates of 0, 0.2, 0.4, and 0.8 kgm�2. Wheat straw wascollected from the field following the wheat harvest. The strawwasair-dried and cut or broken to lengths of less than 30 cm, the pur-pose of this treatment was because under the field condition thewheat crop was harvested using the harvester machine that pro-duce wheat straw ranging from 25 to 30 cm length.

The soil without mulch (0 kgm�2) served as the control. Prior toa simulated rainstorm, the boxes were moved into predeterminedrandomized positions, in two rows of six boxes each, under therainfall simulator and the slopes were adjusted to the designatedgradient. Three slope gradients were studied: gentle (5�, 8.7%),moderate (15�, 26.8%) and steep (25�, 46.6%).

Each rainfall experiment consisted of 60mm of rainfall, whichensured that a dry soil layer would be maintained in the bottom ofthe box to avoid potential drainage problems and the pot effect.Two rainfall intensities were studied (90 and 180mmh�1), whichwere typical of the more intense rainstorms on the Loess Plateau.Three rainstorms were used for each rainfall intensity, and eachtreatment was replicated three times. During each rainstorm, allthe runoff was collected in buckets at intervals equivalent to 3mmof rain depth, i.e., 20 samples were collected from each box (Fig. 2).Following the rainstorm, the buckets containing the runoff waterand sediments were weighed. Sediments were allowed to settleovernight and clear water was removed. The wet sediments weredried at 105 �C and weighed to give the soil loss per 3-mm raindepth interval. Runoff volume was calculated as the difference inmass of the bucket containing the sediment and water and the sumof the tare mass of the bucket and the mass of the sediment. Runoffvolumewas used to calculate the runoff depth (mm) and runoff rate(mm h�1). Infiltration amount (mm) was estimated from the dif-ference between the rainfall depth, adjusted to the projected areaof the box, and the runoff depth.

3. Statistical analysis

The experiment used a 3 Х 3 Х 3 Х 2 Х 4 factorial design (3 soils,3 slopes, 3 replicates, 2 rainfall intensities, and 4mulch treatments).The runoff and soil loss datawas tested for normality and then one-way analysis of variance was used to determine the mean effect ofthe treatments and their interactions. A Tukey HSD post-hoc testwas used to separate between means at probably level of 5%

4. Results and discussion

4.1. Mulch rate and surface runoff

The ANOVA (Table 2) indicated that mulch rate significantlyaffected the amount of runoff (P< 0.01). Soil type, slope gradientand rainfall intensity, as well as some of their interactions, alsosignificantly affected the runoff amount generated by a rainstorm of60mm depth (P< 0.01).

Interactions between variables (Table 2) gave the effect of soil

Table 1Basic properties of the soils used in the study.

Soil type Cation exchange capacity (mmol kg�1) Sodium exchangeable percentage (%) Organic matter content (%) Particle size distribution

Sand (%) Silt (%) Clay (%)

Silt loam 62.2 3.9 0.5 17.6 66.3 16.2Clay loam 110.6 4.2 0.8 24.9 43.4 31.8Loam 75.2 5.4 0.9 28.3 45.4 26.3

A.E. Rahma et al. / International Soil and Water Conservation Research 7 (2019) 335e345 337

type, slope, rainfall intensity and mulch rate on soil loss and runoff.There were two star significant differences (P� 0.01) dependentvariables interactions between two variables: there were two starsignificant differences (P� 0.05) for soil type x mulch rate, soil typex slope, soil type x rainfall intensities, mulch rate x slope, mulchrate x rainfall intensity, slope x rainfall intensity and soil type xrainfall intensities in the soil loss and runoff. There were no sig-nificant difference (P� 0.05) for interactions of soil type x slope,soil type x rainfall intensity, mulch rate x rainfall intensity, slope xrainfall intensity on runoff exceptmulch rate, soil type, mulch rate xslope on runoff. There were two star significant differences(P� 0.01) dependent variables. Interactions involving three ormore variables: there were no significant differences (P� 0.05) forall the interactions. This means that as the number of interaction

increases the significant differences decreases from (P� 0.01) to(P� 0.05). Where there was no significant difference (P� 0.05) forsoil loss, it was also not significant (P� 0.05) for runoff and infil-tration. Where variables interactions was significant (P� 0.05) forsoil loss, it was also significant for runoff that where there was nosignificant difference (P� 0.05) for soil loss, it was also not signif-icant (P� 0.05) for runoff and where variables interactions wassignificant (P� 0.05) for soil loss, it was also significant for runoff.This confirms findings by Adekalu, (2006) that where there was nosignificant difference (P� 0.05) for soil loss, it was also not signif-icant (P� 0.05) for runoff and where variables interactions wassignificant (P� 0.05) for soil loss, it was also significant for runoff.

The total runoff amount generated by a 60-mm rainstormdecreased with increasing mulch rate for all soil types under both

Fig. 1. Rain fall simulator.

Fig. 2. Experimental equipment used to collect data under rainfall simulator.

A.E. Rahma et al. / International Soil and Water Conservation Research 7 (2019) 335e345338

rainfall intensities (Table 3 and.4). In the case of the zero mulch rate(control), the order of the soil types producing decreasing amountsof runoff was the same for each slope and rainfall intensity: clayloam> loam> silt loam. For the three slopes, the ranges of thepercentages of water lost as runoff during a 60-mm rainstorm at90mmh�1 were 67%e81%, 59%e67%, and 22%e38% for the clayloam, loam and silt loam soils, respectively; when the rainfall in-tensity was 180mmh�1 the corresponding ranges were 81%e94%,68%e81%, and 42%e49%.

The greater amounts of runoff generated from the soils wheremulch cover was absent as compared to mulch covered soils indi-cate that a greater degree of surface sealing occurred. When a soilsurface is exposed to raindrop impact the amount of runoff reflectsthe degree of surface sealing. The hydraulic conductivity of thesurface seal limits infiltration.

In un-mulched soil, the runoff was significantly affected byrainfall intensity and slope gradient, i.e., runoff increased withincreasing rainfall intensity and slope gradient (Tables 3 and 4). Theminimum and maximum runoff of 12.90 and 56.69mmm�2 for theclay loam, loam and silt loam soils, respectively, were recordedduring the lowest rainfall intensity under the lowest slope andhighest rainfall intensity with the steepest slope, respectively. Slopeis an important factor influencing the runoff generation process(Bracken & Croke, 2007; Mu et al., 2015). It strongly affects thewater storage on the soil surface (Onstad, 1984; Kamphorst et al.,2000). Therefore, changes in slope gradient could alter the runoffprocess. The results show that the effect of slope on the runoff

varied with the rainfall intensity (Tables 3 and 4). Runoff signifi-cantly (a¼ 0.05) increased with increase in slope gradient duringall rainfall intensities in un-mulched soil. However, an increase inslope gradient from 5� to 15� had a non-significant effect on runoffunder the rainfall intensities of 180mmh�1, because during higherrainfall intensities, the time required for runoff generation was tooshort to reflect the difference between slopes (Cao, Liang, Wang, &Lu, 2015). Whereas, the same slope gradient (increase from 5� to15�) with a rainfall intensity of 90mmh�1 had a significant effecton runoff. The effect could be due to the permeable conditions ofsoil, which absorbed more water during lower rainfall intensity,and delayed runoff on 5� slopes. The effect of increasing rainfallintensities at a specific slope level was also compared and differentpatterns of runoff generationwere recorded at different slope levels(Tables 3 and 4). The runoff losses significantly (a¼ 0.05) increasedwith an increase in the rainfall intensities from 90 to 180mmh�1 atall slope levels for the clay loam, loam and silt loam soils., Whenrainfall begins, surface depressions progressively overflow and areconnected to nearby depressions resulting in overland flow(Onstad, 1984; Darboux, Davy, Gascuel-Odoux, & Huang, 2002;Antoine, Javaux, & Bielders, 2011; Yang & Chu, 2013). This processstarts when the infiltration capacity during a rainfall event is lowerthan the rainfall intensity (Yang & Chu, 2013).

Once formed, sealed soils generally have lower hydraulic con-ductivities and infiltration rates and have higher shear strengthsthan unsealed soils although this very much depends on the type ofseal in place. These conditions combine to increase runoff and in-fluence local erosion processes (Assouline & Ben-Hur, 2006;McIntyre, 1958).

In this study, the clay loam was the most susceptible to surfacesealing while the silt loam was the least susceptible resulting inmore runoff from the former soil than from the latter. Our resultsconfirmed the previous findings by Le Bissonnais et al. (2007) thatthe increase in clay content could largely explain the increase in soilaggregate stability when organic C contents were low. The sus-ceptibility of the three soil types to surface sealing was controlledby a number of factors. Surface sealing occurred because soil ag-gregates were first broken, the resulting loose particles entered thesoil pores and inhibited the flow of infiltrating water, while theporosity of the surface layer was further reduced by compactiondue to raindrop impact (McIntyre, 1958). Two physical processeswould have occurred in these experiments that resulted in aggre-gate breakdown, i.e., raindrop impact and slaking. Slaking occurredbecause of the explosion of air entrapped under pressure withininitially dry soil aggregates during rapid wetting, which occurred inthese experiments because of the high rainfall intensities used(Quirk & Panbokke, 1962; Shainberg, Mamedov, & Levy, 2003; Hanet al., 2016; Almajmaie, Hardie, Acuna, & Colin, 2017). Aggregatebreakdown and surface sealing were enhanced by clay dispersion

Table 2Analysis of variance for the runoff and total soil loss data generated in the rainfallsimulator.

Sources DFa F-value

Soil loss Runoff

Soil 2 22.9** 123.4**Mulch 3 25.8** 193.4**Slope 2 7.6** 21.4**Rainfall intensity 1 11.6** 5.5**Soil * Mulch 6 8.8** 11.2**Soil * Slope 4 3.6** 1.0ns

Soil * Rainfall intensity 2 8.9** 0.5 ns

Mulch * Slope 6 1.6* 5.0**Mulch * Rainfall intensity 3 4.7** 0.7 ns

Slope * Rainfall intensity 2 2.8* 0.2ns

Soil * Mulch * Slope 12 0.9ns 3.1 **Soil * Mulch * Rainfall intensity 6 2.7** 0.1ns

Mulch * Slope * Rainfall intensity 4 0.5ns 1.6ns

Soil * Slope * Rainfall intensity 6 1.7* 0.9 ns

Soil * Mulch * Slope * Rainfall intensity 12 0.7ns 1.4 ns

a DF, degrees of freedom; ns, nsdnot significant; *, ** Significant at 5% and 1%,respectively.

Table 3Total surface runoff (mm) from three soils on different slopes under different mulchrates during a 60-mm rainstorm with a rainfall intensity of 90mmh�1

Soil series Slope Mulch rate (kg m�2)

0 0.2 0.4 0.8

Silt loam Gentle 12.9 (0.44)a 10.61 (0.26) 1.01 (0.05) 1.00 (0.02)Moderate 19.42 (0.56) 11.45 (0.36) 2.03 (0.08) 1.10 (0.05)Steep 22.71 (0.63) 13.03 (0.52) 4.63 (0.31) 2.50 (0.05)

Clay loam Gentle 40.51 (1.3) 30.57 (2.23) 8.58 (0.34) 6.93 (0.23)Moderate 44.61 (1.12) 36.83 (2.30) 10.74 (0.76) 8.71 (0.67)Steep 49.7 (1.39) 40.29 (2.24) 12.19 (0.95) 9.97 (0.67)

Loam Gentle 35.09 (1.58) 25.24 (1.17) 5.82 (0.61) 3.83 (0.41)Moderate 38.83 (1.86) 27.51 (1.28) 6.41 (0.20) 4.73 (0.82)Steep 40.22 (1.17) 30.29 (1.36) 7.65 (0.15) 5.85 (0.92)

a Values are means with (standard deviation).

Table 4Total surface runoff (mm) from three soils on different slopes under different mulchrates during a 60-mm rainstorm with a rainfall intensity of 180mmh�1

Soil series Slope Mulch rate (kg m�2)

0 0.2 0.4 0.8

Silt loam Gentle 25.15 (1.61)a 12.63 (1.10) 6.13 (0.86) 5.53 (0.38)Moderate 27.09 (1.13) 14.93 (1.32) 8.29 (0.49) 7.80 (0.44)Steep 29.45 (1.16) 16.37 (1.12) 9.10 (0.58) 8.75 (0.49)

Clay loam Gentle 48.74 (2.74) 40.47 (1.08) 20.24 (0.76) 9.45 (0.43)Moderate 50.46 (2.95) 43.01 (1.41) 26.70 (0.80) 12.93 (0.59)Steep 56.69 (3.12) 45.13 (1.45) 29.64 (0.85) 14.43 (0.79)

Loam Gentle 40.44 (2.67) 33.53 (1.51) 8.47 (0.53) 7.91 (0.13)Moderate 43.16 (2.74) 36.58 (1.22) 9.68 (0.59) 9.43 (0.22)Steep 48.30 (2.86) 40.77 (1.14) 11.60 (0.71) 10.14 (0.35)

a Values are means with (standard deviation).

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released by a physicochemical process due to the low electrolytecontent of rainwater (Agassi et al., 1981). The three studied soils allhave relatively high silt contents ranging from 43% to 66% (Table 1).Soils high in silt usually have weak aggregates that are more sus-ceptible to aggregates breakdown and consequently to surfacesealing. In addition, the soil aggregates were weak due to loworganic matter contents that were all less than 1%. Although claycontent often increases aggregate stability, the presence of higheramounts of clay also provide a source of free clay particles afterdispersion that enter the soil pores and inhibit infiltration. There-fore, sealing occurred to a greater degree in the soils withmore clay.

In mulched soil, the runoff losses increased with increasingrainfall intensity and the slope gradient. The results, inTable (Tables 3 and 4), indicate that the maximum runoff of 45.13.mm$m�2 was recorded for the rainfall intensity of 180mmh�1 atthe 25� slope level under clay loam soil. Straw mulch significantly(a¼ 0.05) reduced runoff volume as compared with the un-mulched treatments (Fig. 3), indicating that a portion of rainfallwas either infiltrated into the soil or absorbed by the straw coveredsurface. The air-dried straws can to absorb water up to 4.8 times itsoriginal weight (Wu, Chen, & Tsou, 1995). Moreover, the conser-vation effect of strawmulch reduced with increasing slope gradientduring higher rainfall intensities (3a, b and c). These trends indicatethat a reduction in runoff losses decreased with an increase inrainfall intensity in mulch treatments at a steep slope.

Mulch rate had a significant effect on runoff. Mean runoff losseswere 34%, 25%, 10% and 6% under mulch rates of 0, 0.2, 0.4 and0.8 kgm�2, respectively, under the 90mmh�1 rainfall; while under180mmh�1, the corresponding runoff losses were 41%, 32%, 18%and 10%. When mulch intercepts the raindrops, aggregate break-down due to raindrop impact is reduced. With increased mulchrates, the degree of cover protecting the soil surface from raindropimpact increased in this study. The runoff reductions could beattributed to the protection provided by the mulch against thedirect impact of raindrops, promoting the dispersion of the kineticenergy of the raindrops, preventing the destruction of soil aggre-gates and the compaction of the soil surface layer (Gholami et al.,2013). Consequently, infiltration rates were maintained to agreater degree and, for a given rainfall intensity and slope, therunoff was reduced (Bajracharya & Lal, 1998). However, mulch canalso reduce runoff by increasing surface roughness and enhancinginfiltration (Cook et al., 2006; Jord�an et al., 2010; Montenegro et al.,2013a, b; Zonta et al., 2012; Shi et al., 2013). Increasing the slopegradient and rainfall intensity tended to increase the amount ofrunoff generated under each treatment. Similar findings have beenreported by other researchers, who also observed that increasingslope gradient was the important factor in runoff losses even in thepresence of mulch cover (Won, Choi, Shin, Lim, & Choi, 2012;Adekalu et al., 2006). The degree of surface sealing is a balancebetween seal forming processes and seal destruction processes

Fig. 3. Reduction in water loss due to mulch cover for three soil types form different slopes under two rainfall intensities (90 and 180 mm h-1 represented by filled and unfilledsymbols, respectively).

A.E. Rahma et al. / International Soil and Water Conservation Research 7 (2019) 335e345340

(Poesen, 1987)ntensity can both increase the frequency of raindropcompaction or the degree of slaking (forming processes) and soildetachment (a destruction process). In this study, seal formingprocesses may have been enhanced by the increase in rainfall in-tensity more than the seal destruction processes.

Reductions in water losses due to mulch cover may be repre-sented by percentage reductions in runoff using the control as abaseline (Fig. 3). Fig. 3 indicates clearly that although runoff wassubstantially reduced by amulch rate of 0.2 kgm�2 for all soil types,rainfall intensities and slopes, it was further reduced substantiallyby a further increase in mulch rate. For the silt loam (Fig. 3a) andthe loam (Fig. 3c) soils, applying 0.4 kgm�2 effectively reducedrunoff and a further increase of mulch did not greatly enhancerunoff reduction. Therefore, a mulch application of 0.4 kgm�2

would be optimal for these soils. However, for the clay loam soil,while the mulch rate of 0.4 kgm�2 would be optimal under the90mmh�1 rain, it was not for the higher intensity rainstorm.Therefore, for clay loams likely to be subjected to rainfall intensitiesin the order of 180mmh�1, a higher rate of mulch (0.8 kgm�2)would be optimal in order to reduce water losses.

However, some scholars (Won et al., 2012) reported that strawmulch (600 g $ m� 2) had no runoff during rainfall of 30mm $ h�1

on both 10� and 20� slopes, and negligible runoff in a simulation of60mm $ h�1 on the 10� slope. Our present study also shows thatmulching decreases runoff losses (Fig. 3). However, these dataindicate that the application of straw mulch to 0.4 kgm �2 duringlow rainfall intensity may not be economical. Indeed, runoff lossesfrom un-mulched soil during lower rainfall intensity were negli-gible compared to those under higher rainfall events. During lowrainfall intensities, more rainfall is intercepted by high mulch rate,and also partly influences soil water conservation (Li et al.,2005).However, there was threshold mulch thickness to avoidexcessive water interception during low-intensity rainfalls (P�erez,2000). Although the magnitude of reduction in runoff lossesdecreased with an increase in slope during high-intensity rains, thereductions were significant. These results conclusively demonstratethe effectiveness of straw mulch in reducing runoff losses.

4.2. Mulch rate and soil loss

Mulch reduced soil losses from all soil types on the differentslopes and under different rainfall intensities (Table 5 and 6). Soillosses increased with increasing slope and rainfall intensity similarto the runoff increases, and were reduced by increasing mulchrates. In part, when the runoff amount increases, greater soil lossesare likely due to an increased capacity for sediment transport. Inaddition, increasing the slope gradient or the rainfall intensity re-sults in higher runoff rates and faster flow velocities, which have ahigher capacity for soil detachment and a higher carrying capacity.

Mulch reduces flow velocity of the runoff regardless of the runoffrate (Rahma et al., 2013), which generally reduces the carryingcapacity and soil losses, even though mulch also increase rough-ness, which may increase detachment (see Table 6).

Mulch also reduces soil losses by reducing detachment due toraindrop splash. Furthermore, soil detachment and transport areenhanced when the particles are smaller, while water stable ag-gregates are larger and harder to erode. Under mulch, the reductionin raindrop impact results in less aggregate breakdown, althoughslaking is still a factor, leading to fewer smaller or erodible particles(Shi et al., 2013; Sirjani and Mahmoodabadi., 2014).

Mulch also reduced and delayed rill formation, in particular bydecreasing runoff velocity and its sediment transport capacity(Montenegro et al., 2013a, b; Shi et al., 2013). Also, by protecting thesoil surface from the direct impact of raindrops, mulching reducedsoil detachment by splash erosion and the amount of soil availablefor mobilization by runoff (Cerd�a et al., 2016; Foltz &Wagenbrenner, 2010; Gholami et al., 2013; Groen & Woods,2008; Montenegro et al., 2013a, b; Pannkuk & Robichaud, 2003;Prats et al., 2012, 2014, 2015, 2017; Robichaud et al., 2013).

Moreover, the presence of a water layer on the soil surfacecontrols the detachment rate (Kinnell, 2005). As a result, thedissipation of raindrop kinetic energy greatly influences thedetachment and transport processes in rain impacted flows, andmore of the raindrop energy is dissipated in the water layer as flowdepth increases, leading to a reduction in the soil erosion rate(Kinnel, 2010) Thinner flow depth on the soil surface exposes theaggregates to raindrop impact and exacerbates the soil loss losseson steep slopes.

The order of soil erodibility followed that of the soil clay content,i.e. soil losses were in the order clay loam> loam> silt loam. Due tothe high silt contents and low organic matter contents, all the soilshad weak aggregate stability, so the potential for clay dispersionand removal was likely a factor in the soil erodibility.

Similar to assessing the efficacy of mulch in reducing waterlosses, the efficacy in reducing soil loss can be assessed from thepercentage reductions in soil loss using the control as a baseline(Fig. 4).

Fig. 4 indicates clearly that the relation of mulch rate to soil lossreduction was different for different soil types and was alsodifferent from the relation to water loss reduction shown in Fig. 3.In the case of the silt loam, a mulch rate of 0.2 kgm�2 would besufficient to prevent 60%e80% of the soil losses, depending on therainfall intensity and slope conditions (Fig. 4a). However, a further10% reduction in soil losses could be achieved by increasing themulch rate to 0.4 kgm�2 but there would be no clear benefit fromincreasing the mulch rate to 0.8 kgm�2. Furthermore, since the useof mulch for soil conservation prevents its use for other things,doubling the mulch rate to achieve a soil loss reduction of 10% may

Table 5Total soil loss (kg m�2) from three soils on different slopes under different mulchrates during a 60-mm rainstorm with a rainfall intensity of 90mmh�1

Soil series Slope Mulch rate (kg m�2)

0 0.2 0.4 0.8

Silt loam Gentle 0.15 (0.05)a 0.05 (0.00) 0.02 (0.00) 0.01 (0.00)Moderate 0.26 (0.07) 0.08 (0.00) 0.03 (0.00) 0.03 (0.00)Steep 0.71 (0.08) 0.12 (0.01) 0.04 (0.00) 0.04 (0.00)

Clay loam Gentle 1.19 (0.13) 0.50 (0.05) 0.20 (0.00) 0.06 (0.00)Moderate 1.64 (0.14) 0.70 (0.08) 0.30 (0.06) 0.09 (0.00)Steep 2.43 (0.03) 0.99 (0.01) 0.44 (0.01) 0.16 (0.02)

Loam Gentle 0.52 (0.01) 0.12 (0.00) 0.06 (0.00) 0.03 (0.00)Moderate 0.64 (0.01) 0.23 (0.05) 0.07 (0.00) 0.04 (0.00)Steep 0.88 (0.01) 0.41 (0.08) 0.09 (0.00) 0.05 (0.02)

a Values are means with (standard deviation).

Table 6Total soil loss (kg m�2) from three soils on different slopes under different mulchrates during a 60-mm rainstorm with a rainfall intensity of 180mmh �1.

Soil series Slope Mulch rate (kg m�2)

0 0.2 0.4 0.8

Silt loam Gentle 0.20 (0.50)b 0.08 (0.02) 0.04 (0.01) 0.02 (0.00)Moderate 0.38 (0.61) 0.09 (0.001) 0.06 (0.01) 0.05 (0.00)Steep 0.95 (0.93) 0.21 (0.010) 0.09 (0.00) 0.06 (0.00)

Clay loam Gentle 2.07 (0.19) 1.50 (0.12) 1.30 (0.00) 0.08 (0.00)Moderate 5.22 (0.20) 1.95 (0.14) 1.60 (0.05) 0.23 (0.01)Steep 7.95 (0.40) 2.52 (0.17) 1.71 (0.09) 0.43 (0.04)

Loam Gentle 9.70 (0.42) 0.53 (0.05) 0.08 (0.00) 0.09 (0.00)Moderate 1.16 (0.15) 0.72 (0.06) 0.18 (0.00) 0.14 (0.01)Steep 1.21 (0.17) 0.90 (0.10) 0.22 (0.04) 0.12 (0.02)

a Values are means with (standard deviation).

A.E. Rahma et al. / International Soil and Water Conservation Research 7 (2019) 335e345 341

not be considered to be worthwhile.However, that decision is best left to local land managers. For

the loam soil (Fig. 4c), applying 0.2 kgm�2 would not be sufficientto substantially reduce soil losses for many of the intense rain-storms on the steeper slopes, so that an application of 0.4 kgm�2

would be of greater benefit than in the silt loam case. For the clayloam, 0.4 kgm�2 may be optimal under the 90mmh�1 rain, but notfor the higher intensity rainstorm. Therefore, for clay loam soilslikely to be subjected to rainfall intensities in the order of180mmh�1, a higher rate of mulch (0.8 kgm�2) would be optimalin order to reduce water losses. Furthermore, if the benefits of afurther increase of 10% soil reduction are worth increasing themulch cover from 0.4 to 0.8mmh�1, then this would be the optimalrate under all conditions for the clay loam soil would be0.8mmh�1.

5. Conclusions

The efficacy of applying mulch in order to reduce soil and waterlosses from cultivated soils with three different textures exposed tointensive rainfall conditions was assessed. Applying mulch alwaysreduced water and soil losses as compared to a bare soil withoutmulch. However, the levels of mulch required to reduce soil andwater losses depended on the soil type and the rainfall and slope

conditions. The level could also depend on whether the objectivewas to primarily reduce soil or water losses.

For short slopes (1m), in order to save water, it was found thatthe silt loam and the loam soils should have mulch applied at a rateof 0.4 kgm�2 and a further increase of mulch did not greatlyenhance runoff reduction. However, for the clay loam soil, while themulch rate of 0.4 kgm�2 would be optimal under the 90mmh�1

rain, it was not for the higher intensity rainstorm. Therefore, forclay loams likely to be subjected to rainfall intensities in the orderof 180mmh�1, a higher rate of mulch (0.8 kgm�2) would beoptimal in order to reduce water losses.

For short slopes, in order to save soil, it was found that the siltloam could be treated with as low a mulch rate as 0.2 kgm�2 ifpreventing 60%e80% of the soil losses, depending on the rainfallintensity and slope conditions, was sufficient. However, a further10% reduction in soil losses could be achieved by increasing themulch rate to 0.4 kgm�2 but there would be no benefit fromincreasing the mulch rate to 0.8 kgm�2. For the loam soil, mulch atthe rate of 0.4 kgm�2 was essential in most cases in order to reducesoil losses substantially. For the clay loam, 0.4 kgm�2 may beoptimal under the 90mmh�1 rain, but not for the higher intensityrainstorm. Therefore, for clay loam soils likely to be subjected torainfall intensities in the order of 180mmh�1, a higher rate ofmulch (0.8 kgm�2) would be optimal in order to reduce soil losses,

Fig. 4. Reduction in soil loss due to mulch cover for three soil types form different slopes under two rainfall intensities (90 and 180 mm h-1 represented by filled and unfilledsymbols, respectively).

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as was the case for the water losses from this soil. These optimalvalues would also need to be considered alongside other factorssince the mulch has value if used elsewhere. Hence doubling theoptimal mulch rate for the silt loam soil from 0.2 kgm�2 or the clayloam soil from 0.4 kgm�2 in order to achieve a further 10% reduc-tion in soil loss needs to be assessed in that context.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by National Natural ScienceFoundation of China under Project No. 41230746 and No. 51621061.

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